The present invention is directed to the digital color imaging arts. It finds particular application to a system and method of printing a digital color image and will be described with particular reference thereto. Of course, it is to be appreciated that the invention is also applicable to other environments and applications, such as other digital rendering systems, e.g., image setters, direct-to-plate systems, and video displays.
In commercial printing, a typical electronically-prepared job involves an art director, a professional photographer, a graphic artist, a document composition and layout specialist, a proofing system operator, a printing press operator, and the customer. The art director develops a concept for the document, the photographer shoots a large volume of the best possible pictures of the subject matter, and the customer approves the images and concept of the pieces. The graphic artist draws and colors the complex illustrations, and the document layout specialist assembles the text, graphics, and images into a source file for film making. This file, usually located on a storage disk, is called a source file.
The source file is typically an array of pixel information for each of the imaging colors red, green and blue (RGB). The source file could also be an array of pixel information for each of the imaging colors cyan, magenta, yellow and black (CMYK). Each pixel is represented as an intensity value, from 0 to 255, of each the three colors.
To print the file onto film or paper, the source file is converted into a file of instructions in page description language (PDL). Following this, a digital front end processes or decomposes the PDL file into a contone image of 8 bits per pixel or a byte map. This process or decomposition is known as raster-image processing (RIP). Then, the contone image is sent to a print engine containing a half toner or screen generator. Typically, the half toner renders a raster image for each of the print colors cyan, magenta, yellow and black (CMYK). Each raster image is composed of pixel data of 1 bit/pixel. Thus, each bit is simply an instruction whether or not to place a dot of color at a particular point on an output page.
To aid in the conversion of three-dimensional RGB signals to four-dimensional CMYK signals, a multi-dimensional look-up table is commonly used. The look-up table converts each digital RGB color signal value to a corresponding digital CMYK value before being received by the printer.
A printer which has an ideal dye behavior has a one-to-one correspondence of cyan-to-red, magenta-to-green, and yellow-to-blue. This means that when printed, the cyan ink will only absorb red light, the magenta ink will only absorb green light, and the yellow ink will only absorb blue light. However, printers inherently have a non-ideal dye behavior and therefore have a complex non-linear colorimetric response. Interactions between the cyan, magenta, and yellow inks exist which result in unwanted absorptions of reds, greens, and blues. Even once a printer is calibrated such that a range of input digital CMYK values produce the proper colors, the full spectrum of CMYK values and printed colors is not accurate. In other words, the colors asked to be printed and the actual colors printed are not the same.
This discrepancy arises because the relationship between digital values that drive the printer and the resulting colorimetric response is a complex non-linear function. Modeling the colorimetric response to achieve linearity across the available spectrum usually requires many parameters. Therefore, the relationship between the CMYK values driving the printer and the measured colorimetric values of the resulting printed patch is often not characterizable by a simple function or model. The number of measurements required to characterize the printer adequately, can be 1,000 or more measurements. Typically, a color correction look-up table is built which approximates the mapping between RGB space and CMYK values. More specifically, the color correction look-up table corrects for non-linearities and unwanted absorptions of inks such that the printer prints the true corresponding color.
Each RGB coordinate is typically represented by an 8-bit red value, an 8-bit green value, and an 8-bit blue value. Although the RGB coordinate is capable of addressing 2563 locations, each look-up table for each of the RGB colors is typically partitioned into a smaller size, such as 16xc3x9716xc3x9716 (4096) table locations. The number of table locations is selected based on the desired accuracy of the look-up table compared to the expense of storing a large number of values.
After CMYK values are obtained, they are adjusted using tone reproduction curves (TRCs) to adjust the aesthetic result of the printed image. The TRCs are a set of data defining the input to output relationship for each separation for all possible input values. Typically, the TRCs are represented by look-up tables. By transforming the CMYK values, the color density of the output image, as represented by the output data, is adjusted. In this regard, original documents are created using scanned images. In a digital printing machine, the image processing system can greatly impact the contrast of the output image. To assure high quality at the output printing device, it is desirable to know the contrast of the image that has been scanned so that the TRCs may be adjusted in image processing to reproduce the image with a desired appearance.
One way of obtaining this contrast information is to generate a grey level histogram, which gives an easy to read measure of the image contrast. The image or grey level histogram describes the statistical distribution of grey levels of an image in terms of the number of pixels at each grey level. In other words, the histogram is a representation of the number of pixels within an image that are associated with a certain grey level.
A histogram can be represented graphically with intensity on the horizontal axis from 0 to 255, if an eight-bit per pixel sampling resolution is utilized, and the number of pixels on the vertical axis. Using this graphical representation, a histogram can illustrate whether an image is basically dark or light. It is important to know that when an image is represented by histogram, all spatial information is lost. The histogram specifies the number of pixels of each grey level but gives no indication where these pixels are located in the image. In other words, very different images may have very similar histograms.
Conventionally, when creating a histogram of the image, a digital image processing system samples a document, collects intensity data from the document, and uses this information to determine the document""s background value. In such conventional systems, the computed background value of the document represents the average intensity of the document.
Achieving accurate scanner to printer color accuracy and optimizing contrast are two aspects of digital color image processing. Another aspect of digital color image processing is adjusting a balance of characteristics of the printed image to aesthetically improve the appearance of the reproduced image. Examples of such characteristics of the printed image are the lightness/darkness, midtone contrast, highlight emphasis, shadow emphasis, and color cast of the image.
Current digital imaging machines typically provide operators with the ability to adjust the appearance of the printed image. Generally, digital machines offer control over output lightness and darkness, sharpness, and contrast. Lightness and darkness adjustments, for example, vary hue along a scale from black to white. In addition, operators can adjust the balance between colors and overall amount of color on output copies.
Color and tone corrections to the document may be applied to the images and graphics at a client workstation. These corrections are applied by the graphic artist or layout specialist. More specifically, an operator is able to adjusts the amounts of the three process colors, cyan, magenta, and yellow (CMY)K to shift the color balance or image characteristics of the printed image to a preference. Generally, these factors can only be changed by undergoing raster-image-processing again. This is time consuming (taking up to 15 minutes or in some instances) and, thus, expensive. Nevertheless, greater control by an operator of a printing system is desirable because print appearance is often subjective. Further, it is often difficult to determine the most preferable print appearance without first seeing a printed image.
It would therefore be desirable to provide a printing system operator with the ability to apply tone and color modifications without having to send the job back to another person in a different department, such as a graphic artist or layout/color correction specialist, to make changes. Further, it would be desirable to avoid re-RIPping the document to introduce tone and color modifications to the document. Thus, it is desirable to save an intermediate form of the digital image data that is still modifiable and capable of being converted to device-specific and color-specific half-tones at printer speeds. This would result in improved productivity and reduced cycle time.
In accordance with one aspect of the present invention, a digital image printing apparatus is provided which adjusts image characteristics of a displayed image. An input station generates an input image. An image processing station processes the input image and generates a post-processing contone image therefrom. A user interface provides input signals to adjust at least one of the image characteristics of the post-processing contone image. An image control detects the input signals and adjusts at least one of the image characteristics of the post-processing contone image. A display displays the adjusted post-processing contone image.
In a more limited aspect of the present invention, the image processing station includes a raster-image processor for decomposing a representation of the input image in page description language into the post-processing contone image. Further, the apparatus includes a half-toner for converting the adjusted post-processing contone image into a raster image. Still further, the display is a digital printer for printing the raster image.
In accordance with another aspect of the present invention, a method is disclosed for adjusting image characteristics of a input image that is represented in a page description language (PDL image). The PDL image is converted into a post-processing contone image. A tone-reproduction curve (TRC) is selected based on a user input. The post-processing contone image is mapped into an adjusted post-processing contone image using the TRC. The adjusted post-processing contone image is then displayed.
In a more limited aspect of the present invention, the PDL image is raster-image processed into the post-processing contone image. The adjusted post-processing contone image is half-toned into a raster image. The raster image is printed.
One advantage of the present invention is that it provides a printing system operator with the ability to apply tone and color modifications to a job without having to send it back to another person in a different department or to re-RIP the job.
Another advantage of the present invention is that it allows a customer to quickly apply a plurality of TRC modifications and print differing documents therefor. By examining the plurality of printed documents, a customer can select the best TRC modification for the job.
Yet another advantage is that it allows a user to modify the document and then, any time later, return to the original default setting without re-RIPping.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.